In the manufacture of integrated circuits an important step is the projection of circuit patterns contained on a mask or reticle onto a wafer coated with photoresist which is developed in preparation for further steps in the process. In mask projection systems such as described in U.S. Pat. No. 4,068,947 entitled Optical Projection and Scanning Apparatus and having the same assignee as the present application, it is essential that the pattern on the mask be perfectly focused onto the wafer prior to actual exposure of the mask pattern onto the wafer. Otherwise, the exposed pattern will be out of focus or fuzzy which substantially affects resolution of the pattern lines. Such an automatic focusing system is described in U.S. patent application Ser. No. 146,532 entitled Wafer Auto-Focus and Auto-Flattener now U.S. Pat. No. 4,344,160 and having the same assignee as the present application.
In such systems the entire wafer is exposed substantially at the same time by scanning a light source across the mask and onto the wafer. In such a system it is also important that the wafer be substantially untilted from a defined plane since this also affects focus and resolution. However, inasmuch as the wafer in such systems is exposed substantially all at once, the tilt of the entire wafer is correctable only once, i.e., prior to each exposure.
In step and repeat projection systems this is not the case. In such systems it is feasible to correct focus and tilt for each subfield of wafer prior to exposure of that subfield.
The present invention relates to such a system.